JP3043519B2 - Manufacturing method of high strength hot rolled steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-rolled steel sheet having excellent toughness suitable for increasing the strength of recent ERW pipes, building materials, vehicles and the like.

[0002]

2. Description of the Related Art In recent years, as a hot-rolled steel sheet having excellent strength and toughness, a non-heat-treated steel sheet combining Nb with controlled rolling and controlled cooling has been remarkably advanced in recent years. No. 14609, JP-B-58-7
No. 32 publication. Japanese Patent Publication No. 49-14609 discloses N
Controlled rolling, in which the slab is heated to 1230 ° C. or higher, which is higher than the solution heat treatment temperature of Nb carbonitride, using b-added carbon steel, and then the slab is cooled and rolled at 1100 ° C. This is a method for producing steel. In this method, a slab having a thickness of 200 to 300 mm is heated to a temperature not lower than the solution temperature of Nb carbonitride to a central portion thereof by using a heating furnace for burning gas or heavy oil. Therefore, the austenite crystal grain size of the slab after heating is significantly increased, and the ferrite crystal grains of the steel sheet are refined by the subsequent controlled rolling.
However, in this method, there is a limit in grain refinement, and therefore, there is a limit in improvement in toughness.

Japanese Patent Publication No. 58-732 discloses that a slab is heated to a temperature equal to or higher than the solution heat of Nb carbonitride using Nb-added carbon steel, hot-rolled in a temperature range of 1000 to 1150 ° C., and austenite grains are re-formed. This is a method in which fine grains are formed by crystallization and then rolling is performed at 900 ° C. or lower. This method
Compared to Japanese Patent Publication No. 49-14609, this is an excellent method of making austenite grains fine by rough rolling, but has a drawback that Nb contributes to improvement in strength due to work-induced precipitation during rough rolling.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a new production method for refining the crystal grains of a non-heat treated high-strength hot-rolled steel sheet to which Nb has been added. It is intended to provide excellent hot-rolled steel sheets for electric resistance welded pipes, building materials, vehicles and the like.

[0005]

The gist of the present invention is as follows. C: 0.05 to 0.20%, Si: ≦ 0.60%, M
n: 0.20 to 2.50%, Al: 0.004 to 0.1
0%, Nb: 0.01 to 0.10%, N: 0.002
When heating a continuous cast slab consisting of 0.008%, balance Fe and unavoidable impurities, at least 1100
The temperature range from ℃ to the solution temperature of Nb (C, N) from 1 to 140 ℃ is heated at a rate of 150 ℃ or more per hour, and the retention time at the heating temperature is 5 minutes to 30 minutes. And then hot-rolling the hot-rolled steel sheet.

Hereinafter, the present invention will be described in detail. In order to manufacture a non-heat treated high strength hot rolled steel sheet to which Nb with excellent strength and toughness is added, a small amount of Nb is used as a precipitation hardening element, and a hot / hot rolling / cooling process is performed. It is necessary to refine the crystal grains in the rolling step. Normal,
The steel adjusted to the target component is continuously cast and cooled to the Ar ₃ transformation point or lower, but the thickness of the slab is 200 to 300 mm.
Therefore, its cooling rate is slow. Therefore, Nb
A large precipitate of (C, N) of 0.05 μm or more is deposited. It is said that the precipitates effective for the refinement of the crystal grains by hot rolling are solution-formed and work-induced precipitates at the time of working. Nb still contributes to strength improvement
Is said to be extremely fine Nb (C, N) which is coherently deposited. Therefore, Nb precipitated on the slab
It is necessary that (C, N) is solutionized in a heating step before hot rolling.

[0007] The heating temperature for solution treatment is a temperature T ° of the relationship between the solubility product of the precipitate and the temperature (T: ° K) shown below.
K or more is required. log ₁₀ [% Nb]. [% C +% 0.86N] = 2.26-6,770 / T (1) However, since the austenite crystal grains of the slab heated at this temperature are coarse, Various measures are taken in the subsequent hot rolling and cooling steps to refine the ferrite crystal grains of the steel sheet. That is, various contrivances have been made in the hot rolling process, such as performing large rolling at a low temperature. In the cooling step, some measures have been taken such as increasing the cooling rate in the transformation zone.

By the way, if the austenite crystal grains before hot rolling can be made finer by devising the heating step as well as the contrivance in the hot rolling / cooling step, the ferrite crystal grains in the steel sheet after hot rolling can be made finer and the toughness can be improved. It is extremely effective. As a result of various experiments, the present inventors have found that at least 1100 ° C.
The temperature range from the solution temperature of Nb (C, N) to the heating temperature of 1400 ° C or less is heated at a heating rate of 150 ° C or more per hour, and the holding time at the heating temperature is limited to 5 minutes or more and 30 minutes or less. It has been newly found that the precipitation can form a solution while preventing coarsening of austenite crystal grains.

FIG. 1 shows that 0.15% C-0.30% Si-
1.20% Mn-0.040% Nb-0.020% Al
A 250 mm slab of -0.0040% N steel is heated under different heating conditions, and after maintaining at that temperature for 15 minutes, rolling is started, and the thickness of the finished side plate is 40 mm, and the hot rolling finish temperature is 800.
After hot rolling to a thickness of 6 mm at ℃, the cooling rate after rolling is 15 ℃ / s
ec shows the tensile strength of a steel sheet hot-rolled at a winding temperature of 580 ° C. The experiment was conducted by changing the slab heating conditions at this time by changing the average slab cross-sectional heating temperature and heating rate. According to this, the tensile strength is from 1100 ° C. to 12 ° C.
The temperature increases as the temperature rises to 50 ° C., and hardly changes at 1250 ° C. or higher. The effect of the heating rate is hardly observed at each heating temperature. That is, the Nb (C,
Since the calculated solution temperature of N) is 1240 ° C.,
Above 1250 ° C. where b (C, N) is in solution, the tensile strength hardly changes, indicating that Nb (C, N)
This shows that there is almost no effect of the heating rate on the solutionization of.

FIG. 2 shows the influence of the heating temperature and the heating rate on the Charpy test fracture surface transition temperature (vTrs) of a steel sheet hot-rolled under the same conditions as in FIG. According to this, when the heating rate is 70 ° C./hr, the fracture surface transition temperature increases as the heating temperature increases (the toughness deteriorates),
Despite the same tensile strength above 1250 ° C., the fracture surface transition temperature sharply increases. This is 1
After heating Nb (C, N) to a temperature of 250 ° C. or more, austenite crystal grains rapidly increase after fermentation, and the ferrite crystal grains of the steel sheet are large even after rolling, so that the fracture surface transition temperature is high. On the other hand, when the heating rate is 150 ° C./hr or more, the fracture surface transition temperature increases as the heating temperature increases up to the heating temperature of 1250 ° C.
There is almost no change from ℃ to 1350 ℃. This is because, as for the steel sheet up to the heating temperature of 1250 ° C., the fracture surface transition temperature increased as the tensile strength increased. At higher temperatures, the tensile strength did not change, and the crystal grains due to rapid heating also increased. It does not increase, and thus the fracture surface transition temperature of the steel sheet heated under these conditions does not increase.

FIG. 3 shows the influence of the heating rate on the fracture transition temperature of the steel sheet. This is because the heating rate is 150 ° C /
If it is less than hr, the fracture surface transition temperature does not increase.
These facts show that setting the heating rate to 150 ° C./hr or more is an effective method for producing a steel sheet having high tensile strength and low fracture surface transition temperature (good toughness). ing.

The reasons for limiting the above steel components in the present invention are as follows. C: C is the most effective element for obtaining high tensile strength, and at least 0.05% must be added for this purpose. However, the workability, toughness, and weldability decrease with increasing C, so the upper limit was limited to 0.20%. Si: Although Si is useful as a strengthening element, it is decided to add 0.60% as an upper limit in order to produce steel economically. Mn: Mn is also an effective element for improving strength, and at least 0.20% must be added for this purpose. However, if it exceeds 2.50%, it becomes difficult to produce molten steel, so the upper limit is set to 2.50%.

Al: Al is required to be 0.004% or more in deoxidation, but if it exceeds 0.10%, the crystal grains become coarse and the strength is deteriorated. Therefore, the content of Al is limited to 0.10% or less. Nb: At least 0.01% is required because Nb combines with C and N with a small addition to form Nb (C, N) and strengthens the steel. However, the strength-improving effect is saturated even if added excessively, so the upper limit is set to 0.10%, and the upper limit is limited to the range of 0.01 to 0.10%. N: N is currently inevitably contained in steel, and Nb
Is added to form Nb (C, N) and strengthen the steel, but it is not necessary to add it. N on steelmaking
Is less than 0.002%, so the lower limit is 0.0
02%. Further, if N exceeds 0.008%, the low-temperature toughness of the steel sheet decreases, so the upper limit is 0.008%.
%.

Next, the reasons for limiting the heating conditions are as follows. In order to improve the toughness of the steel sheet, it is necessary to make the crystal grains of the steel sheet fine. By making the austenite crystal grains of the slab at the time of heating fine, the ferrite crystal grains of the steel sheet can also be made fine. The austenite grain size of the slab is affected by the temperature and time maintained and the presence or absence of precipitates that stop grain growth. Accordingly, the heating rate during heating affects the temperature and time to be maintained, and the heating rate 15
If the temperature is less than 0 ° C./hr, the austenite crystal grains of the slab after heating become large, and as a result, the ferrite crystal grains of the steel sheet also become large and the toughness deteriorates. Therefore, the heating rate 150
C / hr or more. In addition, a heating rate of 150 ° C./h
The reason why the heating temperature range to be r or more is limited to 1100 ° C. or more is that, at a temperature lower than that, even if the heating rate is low, the growth of crystal grains is relatively small.

The reason why the heating temperature is set to be equal to or higher than the solution solution temperature of Nb (C, N) is that coarse Nb (C, N) precipitated by slow cooling during casting of the slab is solution-cooled and cooled after hot rolling. This is for obtaining the strength of the steel sheet by fine precipitation, and the upper limit is 1400.
The reason why the temperature is set to ° C. is that at a temperature exceeding the temperature, melting of the surface scale occurs to deteriorate the surface properties of the steel sheet. The reason why the holding time at the heating temperature is set to 5 minutes or more and 30 minutes or less is as follows.
In less than 5 minutes, the solution of Nb (C, N) is insufficient,
If the time exceeds 30 minutes, austenite crystal grains become large during the holding time. The heating method for increasing the heating rate of the slab to 150 ° C./hr or more in the present invention includes a method using induction heating and a method using direct energization, but is not particularly limited. Also, 1100
In a temperature range up to ° C., furnace heating by fuel heating may be performed, and a method using induction heating or direct energization may be used only at higher temperatures. In addition, as a method for manufacturing a steel sheet, any method using a hot plate mill or a thick plate rolling machine in which reverse rolling is performed may be used even when manufacturing with a hot strip mill.

[0016]

EXAMPLE Steel having the chemical components shown in Table 1 was melted in a converter and cast into a 250 mm thick slab by continuous casting.
Regarding the chemical components, steels A and B are Nb-added steels that satisfy the component conditions of the present invention. Table 2 shows the slab heating conditions and the material test results of the steel sheet hot-rolled by the hot strip mill. As the heating method, heating method I: 1100
1100 ° C or higher by gas heating to 150 ° C by induction heating to 150 ° C
/ Heating method II: a method of performing a heating rate of 150 ° C./hr from room temperature to a target temperature by induction heating, a heating method III: a heating method of room temperature to a target temperature of 1100 ° C. by gas heating. In the above, the three methods of increasing the temperature at a rate of 70 ° C./hr were compared. The hot rolling condition is a hot rolling finish temperature of 850.
C., and the winding temperature was 580 ° C. for the Nb-added steel, and the same steel type had the same hot rolling conditions but different heating conditions.

[0017]

[Table 1]

[0018]

[Table 2]

According to this, the steel plates A-1, A-2, A-
3, A-4, A-5, and A-6 satisfy the production conditions of the present invention using steel type A. However, steel plate A-7 uses steel type A, but the holding time during heating is short, and Nb
The solution of (C, N) was insufficient and the desired strength was not obtained. Steel plate A-8 uses steel type A but has a heating rate of 70
It is a comparative example in which vTrs is higher than that of the steel sheet A-1, which is as slow as ° C./hr. The steel sheet A-9 used the steel type A, but was heated at a heating rate of 150 ° C./hr. However, the retention time at the heating temperature was longer than that of the steel sheet A-1 by 40 minutes. Is a high comparative example. The steel sheet B-1 satisfies the manufacturing conditions of the present invention using the steel type B. However, the steel sheet B-2 is a comparative example in which the temperature rise rate is as low as 70 ° C./hr, and vTrs is higher than that of the steel sheet B-1.

[0020]

As described above, according to the present invention, a hot-rolled steel sheet having excellent strength and toughness can be manufactured even with the same steel type and the same hot-rolling conditions, and the effect is industrially large.

[Brief description of the drawings]

FIG. 1: 0.15% C-0.30% Si-1.20% M
n-0.040% Nb-0.020% Al-0.004
The figure which showed the influence of the heating temperature and the heating rate on the tensile strength of the steel plate using 0% N steel,

FIG. 2 is a diagram showing the influence of the heating temperature and the heating rate on the Charpy test fracture surface transition temperature (vTrs).

FIG. 3 is a diagram showing the influence of the heating rate on the Charpy test fracture surface transition temperature (vTrs).

Continuation of the front page (51) Int.Cl. ⁷ identification code FI C22C 38/12 C22C 38/12 (56) References JP-A-10-46258 (JP, A) JP-A-6-271936 (JP, A) JP-A-57-79116 (JP, A) JP 7-35540 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02-8/04 C21D 9/46 -9/48 C21D 6/00, 9/00 C22C 38/00-38/12

Claims (1)

(57) [Claims] 1. C: 0.05 to 0.20% Si: ≤ 0.60% Mn: 0.20 to 2.50% Al: 0.004 to 0.10% Nb: 0.01 to 0. 10% N: 0.002 to 0.008% When heating a continuous cast slab consisting of the balance Fe and unavoidable impurities, at least 1100 ° C. and Nb
The temperature range from the solution temperature of (C, N) to the heating temperature of 1400 ° C. or less is heated at a heating rate of 150 ° C. or more per hour, and the holding time at the heating temperature is 5 minutes or more and 30 minutes or less,
A method for producing a high-strength hot-rolled steel sheet, comprising hot rolling thereafter.